In my own IQ tests (on a 5850 and using the .ini edit), I found, in terms of texture and lighting, that DX10 and DX11 were identical, but much better than DX9. Where DX11 really shines is the realistic softened shadows. DX10 and DX9 had the same uniformly-colored, jagged shadows, whereas the DX11 shadows were soft (darker in the middle, lighter on the edges) and round. It was a subtle touch, but still a nice one. After playing in DX11 and discovering the difference, that particular detail became glaring in DX10, especially when those branches are slapping you in the face as you hump through the jungle (shadows from the other leaves).
I was also able to run the game at 8xAA, 16xAF pretty nicely, especially when ATI Overdrive was maxed out.